(1) Field of the Invention
This invention relates to a power converting circuit and a feedback control circuit thereof, and more particularly relates to a power converting circuit with level or output adjustment and a feedback control circuit thereof.
(2) Description of the Prior Art
FIG. 1 is a circuit diagram of a typical power converting circuit, which is capable of varying the level of output voltage. The power converting circuit includes a control circuit Con, transistor switches SW1, SW2, and SW3, an inductor L, an output capacitor C, and resistors R1, R2, and R3, and is utilized for converting an input power source Vin into an output voltage Vout to drive a load Ld. The resistors R1 and R2 are serially connected between the node for accessing the output voltage Vout and a zero volts reference end, which may be a grounded end, so as to compose a voltage divider for generating an output voltage feedback signal VFB. The resistor R3 and the transistor switch SW3 are serially connected between the junction of the resistors R1 and R2 and a zero volts reference end, and the on/off state of the transistor switch SW3 is switched according to an output switching signal S3 such that the division ratio of the voltage divider can be modulated. The control circuit Con receives the output voltage feedback signal VFB so as to generate the control signals S1 and S2 to respectively control the switching of the transistor switches SW1 and SW2 for tuning the level of the output voltage Vout to a predetermined level. Since the control circuit Con has the level of the output voltage Vout stabilized according to the output voltage feedback signal VFB, the changing of division ratio may lead to different stabilized levels of the output voltage Vout for the purpose of multi-level output.
However, the usage of the transistor switch SW3 for connecting or disconnecting the resistor R3 to the resistors R1 and R2 to rapidly change the ratio of the levels of the output voltage feedback signal VFB and the output voltage Vout, the above mentioned division ratio, may induce severe overshooting or undershooting events in the duration to have the output voltage Vout be modulated to a new level.
In present, some approaches for alleviating the overshooting event and the undershooting event has been disclosed, and the most familiar approaches include minimizing the inductance of the inductor L to reduce the power stored in the inductor L, increasing the capacitance of the output capacitor C to slow down the changing of the output voltage Vout, and using a low-pass filter to reduce ringing effect on the output voltage feedback signal VFB. In addition, FIG. 2 shows a circuit diagram of another typical power converting circuit, which applies a switching speed adjusting signal VS to adjust the switching speed of the transistor switch SW3 form totally on to totally off and vice versa so as to slow down the operation of adjusting the voltage division ratio to alleviate the overshooting and undershooting events.
Even with the above mentioned approaches, the overshooting and undershooting events are still an important issue to be resolved and the object to find out a different solution with broader applications and additional circuit protection is a subject to pursue.